1. Field of the Invention
This invention relates to an objective lens-driving apparatus that controllably corrects errors in tracking grooves or pits formed in an optical recording medium such as an optical disk. The invention also relates to an objective lens-driving apparatus that controllably corrects errors in focusing a beam of light on an optical recording medium such as an optical disk.
2. Description of the Related Art
When signals are recorded onto or reproduced from an optical recording medium through an objective lens, a control is required to position the objective lens in such a way that a light spot is always focussed on the recording grooves or pits accurately. Many of such objective lens-driving apparatuses are of a moving coil type.
However, the moving coil type requires a means for supplying electric power from a stationary part of the apparatus to a coil on a movable part. This power supplying means adversely affects the drive characteristic of the objective lens-driving apparatus, preventing the objective lens from being highly accurately positioned. In addition, the coil requires to be electrically connected to the power supplying means but this electrical connection is rather difficult.
In order to solve these problems, for example, Japanese Patent Hei No. 7-19388 proposes a moving magnet type objective lens driving apparatus.
FIG. 25 is a perspective view, with a partially cutaway view, of a pertinent portion of the conventional apparatus disclosed in Japanese Patent Hei No. 7-19388.
Referring to FIG. 25, flat focusing coils 124a and 124b and flat tracking coils 125a and 125b are wound generally in a flat plane, and securely supported by walls 103a and 103b in opposing vertical planes. The walls 103a and 103b are in one-piece construction with a base 101. A lens holder 110 is rotatably supported on a shaft 109 securely fixed to the base 101, and supports an objective lens 113 and magnets 116a and 116b thereon.
A supporting spring 118 is in the shape of a square loop and maintains the lens holder 110 at its neutral position. A pair of opposing sides of the spring 118 is fixed to the lens holder 110 and another pair of opposing sides engages retaining plates 107 and 107b (only 107b is depicted) to be fixed to the base 101.
When a focus error is corrected, a predetermined electric current is run through the focusing coils 124a and 124b to create an electromagnetic force acting between the current and the magnetic field emanating from the magnets 116a and 116b for focusing control. This electromagnetic force drives the lens holder 110 in a direction perpendicular to the surface of the optical recording medium.
When a tracking error is corrected, a predetermined electric current is run through the tracking coils 125a and 125b so that the lens holder 110 is driven in a direction transverse to the track in the optical recording medium for tracking control.
The aforementioned conventional objective lens driving apparatus requires the spring 118 that supports the lens holder rotatably and holds the objective lens at its neutral position. The use of the spring 118 increases the number of components and makes the assembly operation of the apparatus complex.
A magnetic circuit is formed of only the magnets 116a and 116b and therefore the magnetic field generated by the magnetic circuit is rather weak. As a result, this conventional apparatus makes drive sensitivity low and requires more electric power accordingly.
A small gap between the support shaft 109 and lens holder 110 can cause rattling of the lens holder 110 and/or tilting and vibration of the objective lens 113.